The present invention relates to an automatic coffee percolator comprising a fresh-water reservoir, a continuous-flow heater below said fresh-water reservoir, which heater includes, at the end thereof, a completely closing valve under control of a temperature sensor, and furthermore comprising a percolator element with a filter, underneath said continuous-flow heater, and a receptacle below said percolator element.
Such automatic coffee percolators, wherein the heated water is directed to the filter element by gravity action, have been disclosed, inter alia, by the German utility model No. 75 00 498. In such known automatic coffee percolators percolator the vapor bubbles produced when the water starts to boil in the continuous-flow heater urge water back into the fresh-water reservoir. Only when an externally mounted electric temperature control element has responded and when the water has cooled down, can additional fresh water be charged into the continuous-flow heater. Such vapor is produced especially in the final phase of the water heating process in the continuous-flow heater, which can entail undesired noise.
In contrast with such an automatic coffee percolator, the present invention is directed to an automatic coffee percolator with a particularly simply designed continuous-flow heater which operates very reliably and by which the discharge of hot water or vapor into the fresh-water reservoir is safely avoided; in such an automatic coffee percolator the water flowing through the continuous-flow heater can be stepwise controlled within a narrow temperature range, independently of the pressure of the supplied water.
According to the present invention this object is achieved because said valve at the discharge side of the continuous-flow heater is a pressureless and thermally operating valve which opens in the water discharge direction, and a second valve is provided, at the entry side of said continuous-flow heater. This second valve is of the check type and is open when said fresh-water reservoir contains water.
In such a structure of the continuous-flow heater, the discharge of hot water and vapor into the fresh-water reservoir is safely avoided at temperatures above the boiling point because the check valve at the entry side of the continuous-flow heater closes. In the continuous-flow heater itself, moreover, excessive pressure cannot be produced since the valve which opens in the water discharge direction at the discharge side operates as a safety valve independently of its thermally controlled actuation.
In spite of the provision of an additional valve at the entry side of the continuous-flow heater, the automatic coffee percolator can begin full operation immediately after the fresh-water reservoir is filed because the water can flow toward the continuous-flow heater without being affected by the valve at the entry side and because the valve at the discharge side discharges the water immediately after heating it to its nominal temperature.
It is expedient to design the valve at the discharge side as a ball type valve with a valve stem projecting into the flow channel of the continuous-flow heater. This ensures particularly safe operation and a sturdy construction of the valve at the discharge side of the continuous-flow heater.
To adjust of the control range, it is desirable that the valve seat of the valve at the discharge side of the continuous-flow heater be adjustable. Adjustment of the valve seat in the vertical direction entails a shift of the opening temperature of the valve.
The temperature sensor is preferably a bimetal strip connected to a supporting projection of the valve at the discharge side, with one arm of this strip being connected to the casing of the continuous-flow heater while the other arm is adapted to be freely movable but loosely connected to the supporting projection of the valve. If the first arm of the bimetal strip is permanently attached to the top of the casing of the continuous-flow heater the bimetal strip must be designed in a way that the freely movable arm moves away from the permanently attached arm when temperature rises. If however, the first arm of the bimetal strip is permanently attached to the bottom of the casing, the bimetal strip must be constructed so that the freely movable arm moves toward the permanently attached arm when temperature rises. In the latter case the first arm of the bimetal strip is preferably connected to the heating element at the bottom of the continuous-flow heater.
For a further simplification of the continuous-flow heater structure it is desirable that the valve at the entry side of the continuous-flow heater be a ball valve. It can also be advantageous if the casing of the valve at the entry side of the continuous-flow heater is integral with the fresh-water reservoir.
Finally, it is desirable that the two valves of the continuous-flow heater be so designed that the cross-sectional area of the flow passage of the valve at the discharge side is greater than the cross-sectional area of the valve at the entry side of the continuous-flow heater.
The larger cross-sectional area of the flow passage at the discharge side provides for the quickest possible discharge of the heated water during the start-up period, when all of the water in the continuous-flow heater reaches the proper temperature at the same time. The cross-sectional area of the flow passage at the entry side of the continuous-flow heater, however, is kept small enough that entering cold water, by the time it reaches the discharge side, has reached nominal temperature, if possible, so that changing the position of the valve at the discharge side becomes unnecessary. Such structure of the continuous-flow heater results in control of the PD or proportional plus derivative type.